Hello & welcome,
You are 17 yrs old only, and changes in mood or ups & downs in your age is a normal phenomenon. Also you have also accepted here that you are usually a happy person which is very nice part.
Basically you need to discuss the problem with any responsible person or elder person in your home instead of directly approaching any specialist as this so called depression
is a very common problem.
Statistical study shows each and every person, goes through the stage of depression atleast once in life time. So, don't directly make drugs as your first choice for depression.
Instead, you can visit any clinical hypnotherapist for the same problem & yes depression can be thrown in dustbin with the proper sessions conducted by certified clinical hypnotherapist.
Join any self hypnotism courses, discuss all the day activities with your parents or elder ones before going to sleep every day.
Make yourself busy with constructive work, always talk to confident & strong people, how they survived difficult situations.
Star reading books on different subjects, make it habit.
And lastly, don't over react to any situation or acts by people.. because they are powerless without your reaction.
All the best. Take care.